Prior to the conception and development of the present invention, railway car hand brake mechanisms were well known in the art. They usually include a large, rotatable hand wheel disposed in a vertical plane and mounted on a shaft which, through a gear train, can rotate a chain drum to wind up a chain that is secured at its end remote from the chain drum to the brake rigging of the railway car. As the hand wheel is rotated in one direction, the brakes are applied and rotation of the hand wheel shaft in the opposite direction is prevented by a pawl, which engages a detent wheel on the hand wheel shaft.
The brakes may be released by disengaging the pawl from the detent wheel but this causes rapid rotation of the hand wheel and the gears of the gear train. To avoid rapid rotation of the hand wheel, hand brake mechanisms have been devised which are known as “quick release” mechanisms. Generally these quick release mechanisms include a releasable connecting means between the hand wheel shaft and the gear train. When the connecting means is released, the gears of the gear train rotate rapidly, without constraint by the pawl and detent wheel, but the hand wheel remains stationary.
The Association of American Railroads (AAR) promulgates specifications for the design and operation of railway car hand brake mechanisms. Vertical wheel, gear train, hand brake mechanisms are classified in three categories, namely:                (1) Standard power—provides an average force on the chain of 3350 lbs. with a 125 lb. turning force applied to the rim of a wheel twenty-two inches in diameter.        (2) Intermediate power—provides an average force on the chain of 4475 lbs. with a 125 lb. turning force applied to the rim of a wheel twenty-two inches in diameter.        (3) High power—provides an average force on the chain of 6800 lbs. with a 125 lb. turning force applied to the rim of a wheel twenty-two inches in diameter.        
After setting of the brakes, when the hand brake mechanism is released the gears of the gear train rotate rapidly. This results in the gears and other components being subjected to high forces and to shock, particularly, when the chain becomes fully let-out from the chain drum.
In recent times, the AAR has added a life cycle test to its specifications, and hand brake mechanisms which do not meet the life cycle test cannot be sold for use on railway cars operated in interchange service on United States railroads. The AAR life cycle test for quick release brakes requires that such latter brakes withstand 3000 quick release operations.
To meet such life cycle test requirements, even standard power hand brake mechanisms had to be modified when the life cycle test was adopted. When intermediate power hand brake mechanisms of the type sold prior to the adoption of the life cycle test were subjected to the life cycle test, it was found that the components thereof wore prematurely or were damaged, and it was found to be necessary to add a shock retarder, or absorber, external to the hand brake mechanism, to overcome such wear and damage. Of course, such an external shock retarder is undesirable not only because it is external to the hand brake mechanism but also because of the additional cost and because it requires field modification of the equipment on a railway car if the intermediate power hand brake mechanism is used to replace a standard power hand brake mechanism.
High power hand brake mechanisms sold prior to the adoption of the life cycle test were similarly unable to pass the life cycle test. It should be borne in mind that such high power brake mechanisms normally have additional gears to provide the desired force on the chain, and this results in a higher speed of rotation of at least some of the gears during release of the hand brake mechanism.
Although the use of an external shock retarder might have solved the problems with the higher power hand brake mechanism, a change in the AAR specifications would have been required to permit the use of such an external shock retarder. Attempts were made to redesign the high power hand brake mechanism, such as by making it stronger, so that it would meet the life cycle test without the use of an external shock retarder, but the attempts were not successful.
One of the characteristics of railway car brakes with which the invention is concerned is that the force applied to the chain, and hence, the parts of the hand brake, is non-linear and depends on the extent to which the brakes are applied or released. Thus, as the brakes are applied, relatively little force is required to take up the slack in the chain and the brake rigging, but to meet AAR requirements, the final force on the chain must be as set forth above, namely, 3350 lbs. for a standard power brake, 4475 lbs. for an intermediate power brake and 6800 lbs. for a high power brake.
After slack in the rigging is taken up, which may require, for example, 5–15 inches of chain travel, the force on the chain increases exponentially, e.g. from 200 lbs. to the final value, as the brake hand wheel is further turned to set the brakes. In reaching the final value after the slack is taken up, the chain may travel only two or three inches.
Similarly, when the hand brake is released, the chain force decreases exponentially and reaches a relatively small value shortly after the hand brake is released. The aforementioned co-pending application teaches a single stage, double acting cylinder which displaces the same volume of fluid pressure with each stroke regardless of the resulting force in the hand break mechanism. As a result, a partial amount of fluid pressure is being wasted at the beginning of the brake application cycle, where relatively little force is required to take up the slack in the chain. Since the source of said fluid pressure is typically an emergency reservoir having a predetermined volume, less fluid pressure will be available at the end of the brake application to meet chain force requirement.
As it can be seen from the above discussion, it would be advantageous to optimize the consumption of fluid pressure so that less fluid is consumed during the first stage of brake application leaving more fluid available during the critical end stages of brake application when higher pressure is necessary to meet the minimum chain force requirements.